Near-infrared polarization insensitive MEMS tunable metasurface band-pass filter and application thereof

Abstract

The invention relates to the technical field of optical engineering, and discloses a near-infrared polarization insensitive MEMS tunable metasurface band-pass filter which comprises a silicon substrate and a gold structure covering the silicon substrate. The gold structure comprises an upper-layer gold structure and a lower-layer gold structure which are connected through the MEMS, the upper-layer gold structure and the lower-layer gold structure are both square, and the centers of the upper-layer gold structure and the lower-layer gold structure are subjected to square hollow treatment to form a metasurface An FP resonant cavity is formed between the upper and lower gold structures; surface plasmons are generated on the contact surface of the gold structure and the hollow air layer; under the combined action of the FP resonant cavity and the surface plasmon, the metasurface structure realizes transmission of electromagnetic waves at a specific wavelength, and a band-pass filtering effect is achieved. The tunable metasurface band-pass filter provided by the invention is a transmission type metal metasurface filter, a surface plasmon effect is generated on the contact surface of a gold structure and a square hollow-out air layer, two physical mechanisms are coupled to filter a specific wavelength, and the working wavelength can be tuned through an external MEMS structure.

Description

Technical Field

[0001] The present invention relates to the field of optical engineering technology, and in particular to a near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter and applications thereof. Background Art

[0002] In recent years, research on metasurfaces has gradually shifted from static to dynamic tunability. Such tunable metasurfaces can control their optical properties in real time, promising the dynamic modulation of optical degrees of freedom, such as amplitude, phase, and polarization. In the past few years, various materials, such as liquid crystals, graphene, and phase-change materials, have been used to achieve active control of metasurfaces. These materials exhibit distinct tuning mechanisms, providing diverse approaches to metasurface manipulation. However, the intrinsic properties of some materials hinder the expansion of the operating spectral range, while others are incompatible with complementary metal-oxide-semiconductor (CMOS) processes, limiting miniaturization and commercialization. The collaborative design of microelectromechanical systems (MEMS) and metasurfaces has opened up new avenues for dynamic spectral manipulation. MEMS is a microsystem technology that combines microelectronics with mechanical engineering. By integrating micromechanical structures and electronic components on the same chip or substrate, micro- and nanoscale mechanical manipulation can be achieved. This is applicable to terahertz and infrared wavelengths, making MEMS-tunable metasurfaces suitable for diverse applications in various functional devices. At present, the research on MEMS tunable metasurfaces mainly focuses on tunable metalenses, wavefront control, tunable holograms and polarization control.

[0003] Tunable bandpass filters are core components in optical communication systems, biomedical sensing, gas sensing, and high-resolution spectral imaging. Their performance directly affects the sensitivity and integration of optical systems. Metasurfaces provide a new approach to miniaturized filter design. Traditional Fabry-Pérot (FP) cavities rely on multilayer dielectric films to achieve filtering, but their intrinsic polarization sensitivity requires additional polarization control components for non-polarized light scenarios, significantly increasing system complexity and cost. MEMS tunable metasurfaces provide an important approach. However, in existing practices, metal metasurfaces are often used as reflective filters, and transmissive filters are mostly all-dielectric metasurfaces. However, the dielectric structure is easily deformed by stress and has weak dynamic response capabilities. Summary of the Invention

[0004] In order to overcome or alleviate one or more of the above technical problems, the purpose of the present invention is to provide a near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter and its application, wherein the metasurface of the filter has good tunability and is insensitive to polarization.

[0005] The present invention provides the following technical solutions:

[0006] A near-infrared polarization-insensitive MEMS-tunable metasurface bandpass filter comprises a silicon substrate and a gold structure covering the silicon substrate; the gold structure comprises an upper gold structure and a lower gold structure, which are connected via MEMS; the upper and lower gold structures are both square, and the centers are both hollowed out to form a metasurface structure; an FP resonant cavity is formed between the upper and lower gold structures; surface plasmons are generated on the contact surface of the gold structure and the hollowed air layer; under the combined action of the FP resonant cavity and the surface plasmons, the metasurface structure transmits electromagnetic waves at a specific wavelength, achieving a bandpass filtering effect.

[0007] Preferably, the period of the gold structure is 500 nm, the hollowing ratio K of the upper and lower gold structures is in the range of 0.7 to 0.9, and the thickness h is in the range of 100 nm to 300 nm.

[0008] Preferably, the hollowing ratio of the upper gold structure is 0.8, and the thickness h1 is 180 nm; the hollowing ratio of the lower gold structure is 0.9, and the thickness h2 is 150 nm.

[0009] Preferably, by adjusting the voltage on the MEMS connecting the upper and lower gold structures, the distance between the upper and lower gold structures is changed, that is, the cavity length of the FP resonant cavity is changed, and then the optical properties of the near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter are regulated to achieve dynamic tunable filtering.

[0010] Preferably, the operating range of the near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter is changed by adjusting the geometric dimensions of the upper and lower gold structures, including the hollowing ratio and thickness.

[0011] Preferably, the thickness of the silicon substrate is 500 nm.

[0012] On the other hand, the present invention also provides an application of a near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter, characterized in that the above-mentioned near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter is applied to infrared spectrum filtering and infrared wave control.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] The present invention adopts a double-layer metasurface consisting of a silicon substrate and a square hollow gold structure, which not only realizes the function of the FP resonator, but also can realize dynamic tunable filtering through MEMS. The square hollow gold structure ensures that the filter is insensitive to the polarization state of the incident light.

[0015] In the FP resonant cavity, the upper and lower hollow gold structures act as reflectors, forming a resonant cavity that can transmit electromagnetic waves of a specific wavelength. Furthermore, the surface of the designed gold structure generates a surface plasmon effect, further enhancing the electric field strength within the cavity.

[0016] By adjusting the geometric dimensions of the upper and lower gold structures, including the hollowing ratio K and thickness h, the operating range of the filter can be changed. This tunability gives the filter a wider range of application prospects in fields such as spectral filtering and spectral imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A schematic diagram of a near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter provided by an embodiment of the present invention;

[0018] Figure 2 A schematic diagram of the size measurement of the hollow gold structure in the near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter provided in an embodiment of the present invention;

[0019] Figure 3 A spectrum diagram of the tunable range provided by an embodiment of the present invention;

[0020] Figure 4 The intracavity electric field diagram provided by the embodiment of the present invention;

[0021] Figure 5 This is a polarization-insensitive spectrum diagram provided by an embodiment of the present invention.

[0022] Figure 6 Spectra at different incident angles provided by the embodiment of the present invention.

[0023] In the picture:

[0024] Silicon substrate 1; upper gold structure 21; lower gold structure 22; FP resonant cavity 3. DETAILED DESCRIPTION

[0025] The present invention is described in detail below with reference to the embodiments and accompanying drawings. However, it should be understood that the embodiments and accompanying drawings are merely exemplary descriptions of the present invention and do not constitute any limitation on the scope of protection of the present invention. All reasonable variations and combinations within the scope of the inventive concept of the present invention fall within the scope of protection of the present invention.

[0026] The present invention will be further described below with reference to the accompanying drawings.

[0027] Example 1

[0028] Refer to the attached Figure 1 The near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter provided in this embodiment has the following structure:

[0029] It includes a silicon substrate 1 and a gold structure covering the silicon substrate 1. The gold structure includes a double-layer metasurface composed of a lower gold structure 22 below and an upper gold structure 21 above. The upper and lower gold structures are connected by MEMS. This structure is called a metasurface structure. The upper and lower gold structures are both square hollowed-out metal gold films. The middle of the films are square hollowed out, and they are in the shape of a "U" when viewed from above. The square structure is isotropic, making the filter insensitive to the polarization direction of the incident light. The hollowing ratio is defined as the ratio of the hollow side length D to the period P. The cavity between the upper gold structure 21 and the lower gold structure 22 constitutes the FP resonant cavity 3, where FP is Fabry-Pero. Under the joint action of the FP resonant cavity and the surface plasmon, the metasurface structure realizes the transmission of electromagnetic waves at a specific wavelength, achieving a bandpass filtering effect.

[0030] Specifically, the silicon substrate 1 has a thickness of 500 nm, and the gold structure has a period of 500 nm (this embodiment only applies to a period of 500 nm. A smaller period reduces the tuning range of the filter and reduces transmittance, while an increased period reduces the half-width, lowers resolution, and degrades performance. Therefore, 500 nm was selected based on comprehensive considerations). The upper gold structure 21 has a hollowing ratio of 0.8 and a thickness h1 of 180 nm. The lower gold structure 22 has a hollowing ratio of 0.9 and a thickness h2 of 150 nm. The upper and lower hollowed gold structures act as reflectors, forming an FP resonant cavity 3 that can transmit electromagnetic waves of a specific wavelength.

[0031] The square design of the gold structure makes the filter insensitive to the polarization angle of the incident light.

[0032] By using MEMS to adjust the distance between the two layers of gold structure, that is, to adjust the cavity length of the FP cavity, and then control the optical properties of the filter, dynamic tunable filtering is achieved.

[0033] By adjusting the geometric dimensions of the upper and lower gold structures, including the hollowing ratio K (0.7-0.9) and thickness h (100nm-300nm), the center wavelength and operating range of the filter can be changed.

[0034] The resonance condition of FP resonant cavity 3 is 2ndcosθ = mλ, where n is the refractive index of the cavity medium, d is the cavity length, θ is the propagation angle of light within the cavity, λ is the wavelength of light, and m is an integer, known as the resonance order. When light enters the metasurface FP resonant cavity 3, some of the light is reflected, while the rest enters the cavity. The light within the cavity reflects back and forth between the two mirrors, forming multiple reflections. These reflected light rays interfere with each other, forming a specific resonance mode.

[0035] The contact surface between the gold structure and the hollowed-out air layer generates a surface plasmon effect, enhancing the electric field within the cavity. The resonant frequency of the surface plasmon satisfies a specific dispersion relation, which can be controlled by adjusting the size, shape, and dielectric environment of the metal structure. In the resonant state, the surface plasmon can generate a strong electromagnetic field enhancement near the metal surface, and this enhancement effect is very significant at the nanoscale.

[0036] The FP resonant cavity 3 and the surface plasmon effect jointly determine the electric field distribution in the cavity, which in turn affects the filtering performance of the filter.

[0037] To verify this process, Lumerical FDTD Solutions software was used to verify the near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter proposed in this embodiment. The implementation steps are as follows:

[0038] Design as attached Figure 2 In the metasurface structure shown, the thickness of the silicon substrate 1 is 500 nm, the period of the gold structure 2 is 500 nm, the hollowing ratio of the upper gold structure 21 is 0.8 and the thickness is 180 nm, and the hollowing ratio of the lower gold structure 22 is 0.9 and the thickness is 150 nm.

[0039] The incident light is set to be along the z-axis direction, that is, vertically incident near-infrared (1um-1.6um) non-polarized light.

[0040] A transmittance monitor is set to monitor the transmission curve of the incident light after passing through the metasurface structure, and an electric field monitor is set to observe the electric field distribution inside the structure.

[0041] Adjust the air gap g between the upper and lower gold structures to change the cavity length of the FP resonant cavity 3, and thus change the operating wavelength of the embodiment. Change the g value from 320nm to 480nm, and draw a curve every 20nm. The final transmittance curve is as follows Figure 3 shown.

[0042] The process of changing the g value is regarded as the active tuning effect of the MEMS structure on the metasurface. When the g value increases from 320nm to 480nm, the working wavelength of the embodiment is red-shifted, as shown in FIG. Figure 3 As shown, the operating wavelength moves from 1039nm to 1304nm, the tuning range is 265nm, and the central wavelength transmittance is greater than 0.8, maintaining good filtering performance.

[0043] The working principle of this embodiment can be obtained by Figure 4The electric field distribution diagram illustrates this. The diagram shows a clear electric field enhancement between the two gold layers, caused by the FP resonant cavity 3, as well as surface plasmon effects generated between the gold structure and the hollowed-out air layer. The FP resonant cavity 3 provides a broad tuning framework, while the SPP enhances passband efficiency through nanoscale field localization. When the air gap g is varied, the SPP resonant wavelength shifts synchronously with the FP mode, and the two couple to form a single, high-intensity transmission peak.

[0044] The core advantage of this embodiment is reflected in the full polarization state compatibility. Figure 5 As shown, when the incident light is polarized at any angle, the device's transmission spectrum remains stable, with no shift or attenuation in the resonant peak intensity, fully demonstrating its consistent resonant response to electromagnetic waves of any polarization state. This is attributed to the symmetric structure, which produces the same SPP excitation mode for all polarization states. This feature eliminates the need for polarization control elements in traditional solutions and is suitable for use with incoherent / randomly polarized light sources.

[0045] like Figure 6 The transmission curves for different incident angles are shown. This embodiment demonstrates good angular adaptability. As the incident angle increases, the filter's transmission performance decreases, but has little effect on the operating wavelength. This characteristic, which is not possessed by traditional FP filters, makes it more adaptable to different operating environments.

[0046] The above embodiments are merely preferred embodiments of the present invention, and the scope of protection of the present invention is not limited to the above embodiments. All technical solutions that fall within the scope of protection of the present invention are within the scope of protection of the present invention. It should be noted that improvements and modifications that can be made by a person skilled in the art without departing from the principles of the present invention are also considered to be within the scope of protection of the present invention.

Claims

1. A near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter, characterized in that: The invention comprises a silicon substrate and a gold structure covering the silicon substrate; the gold structure comprises an upper gold structure and a lower gold structure, which are connected via MEMS. The upper and lower gold structures are both square, and the centers are both hollowed out to form a metasurface structure; an FP resonant cavity is formed between the upper and lower gold structures; surface plasmons are generated on the contact surface between the gold structure and the hollow air layer; under the combined action of the FP resonant cavity and the surface plasmons, the metasurface structure transmits electromagnetic waves at a specific wavelength, achieving a bandpass filtering effect.

2. The near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter according to claim 1, characterized in that: The period of the gold structure is 500 nm, the hollowing ratio K of the upper and lower gold structures is in the range of 0.7 to 0.9, and the thickness h is in the range of 100 nm to 300 nm.

3. The near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter according to claim 2, characterized in that: The hollowing ratio of the upper gold structure is 0.8, and the thickness h1 is 180 nm; the hollowing ratio of the lower gold structure is 0.9, and the thickness h2 is 150 nm.

4. The near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter according to any one of claims 1 to 3, characterized in that: By adjusting the voltage on the MEMS connecting the upper and lower gold structures, the distance between the upper and lower gold structures is changed, that is, the cavity length of the FP resonant cavity is changed, and then the optical properties of the near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter are regulated to achieve dynamic tunable filtering.

5. The near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter according to any one of claims 1 to 3, characterized in that: The operating range of the near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter can be changed by adjusting the geometric dimensions of the upper and lower gold structures, including the hollowing ratio and thickness.

6. The near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter according to any one of claims 1 to 3, characterized in that: The thickness of the silicon substrate is 500 nm.

7. Application of a near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter, characterized in that: The near-infrared polarization-insensitive MEMS tunable metasurface bandpass filter as described in any one of claims 1 to 6 is applied to infrared spectrum filtering and infrared wave control.

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